(1) Field of the Invention
This invention relates generally to the field of switch interface circuits and relates more specifically to switch interface circuits consuming minimal power.
(2) Description of the Prior Art
Especially in mobile applications powered by batteries, electromechanical switch operations must only consume minimal power, while maintaining a reliable operation. This is most important for e.g. switches used with smart card applications. In these kind of applications a minimum current drain from a battery is required already during assembly in order to minimize energy loss.
The challenge for the designers of such switch interface circuits is to achieve a system consuming current in the order of magnitude of a few tens of nanoamps whilst the switch is closed, having a minimum impedance when the switch is open to eliminate RFI (Radio Frequency Induced) interference, and to allow for extended switch-closed operation.
There are known patents or patent publications dealing with switch interface circuits:
U.S. Patent (U.S. Pat. No. 5,729,167 to Kujawa et al) discloses a low power consumption switch interface circuit including a current source for providing switch current having a magnitude dependent on a state of a current range signal. An input terminal is provided to conduct the switch current to an external switch. A circuit for detecting a signal present at the input terminal provides a measured pulse when the signal is detected. A current range selector, coupled to the current source, outputs a first state of the current range signal while the measured pulse is outputted from the circuit, and a second state of the current range signal while the measured pulse is not outputted from the circuit. The current source provides the switch current at a first magnitude responsive to the first state of the current range signal, and a second magnitude responsive to the second state of the current range signal.
U.S. Patent (U.S. Pat. No. 6,052,005 to Braun) proposes a low current drain switch interface circuit including an input terminal, coupled to a first terminal of a diode. A voltage follower circuit is coupled to a second terminal of the diode. A current source is coupled between an output terminal of the voltage follower circuit and a power supply terminal. A mechanical switch is coupled to the input terminal. The voltage follower circuit outputs a voltage indicative of a physical state of the mechanical switch.
U.S. Patent (U.S. Pat. No. 3,986,039 to Bovitz) discloses a toggle switch indicator circuit for use with digital logic circuits which may be manually set and cleared by means of a manually operated push-button switch and which incorporates a debounce network and an indicator for displaying the state of the toggle switch at a remote point.
U.S. Patent (U.S. Pat. No. 7,176,588 to Yahagi) discloses a starter drive device which drives a starter motor by, upon actuation of a starter switch, turning a starter relay ON and OFF via an electronic control device, wherein the electronic control device includes: an interface circuit; a power source control circuit; a calculation device and a delay circuit; a buffer; an addition circuit; a latch circuit; a first pull up circuit; a second pull up circuit; and wherein, if the power source control circuit has detected a drop of voltage of the power source when the starter motor is being driven, along with stopping the calculation device, the buffer disconnects the calculation device from the latch circuit, and controls the starter relay by operating the driver circuit via the latch circuit based upon the signals from the interface circuit and the second pull up circuit.